The present invention generally relates to a backup system for a utility power grid and, more particularly, is concerned with an uninterruptible power supply (UPS) system and method having a fast-start capability and a low-energy storage requirement.
Present day computer data centers and internet service providers require highly reliable sources of uninterruptible power. Dual fed utility grid systems plus uninterruptible power supplies with back-up generation are utilized to meet the stringent demands for critical power. A conventional UPS system normally includes a DC-AC inverter, transfer switches, an energy storage unit, oftentimes using massive lead acid batteries, and, in many cases, an alternator driven by a heat engine. During a fault in the dual-fed grid source, DC power from the energy storage unit (usually batteries) is converted to the proper AC voltage, current, and frequency and connected to the critical loads via the transfer switches. Further, many of the conventional UPS systems, rated to provide critical power for long periods of time, have a heat engine and an alternator in addition to the battery energy storage unit. In these conventional UPS systems, the battery energy storage unit is required to provide the electrical power to the critical load until the heat engine is cranked and started and the alternator has achieved a stabilized output.
A problem exists, however, in that conventional engine driven alternator UPS systems with 100 kW-1 MW power ratings typically require at least 15 seconds to start the heat engine and alternator and stabilize the electrical output prior to connecting the alternator to the critical load via the transfer switches. During the initial 15 seconds, the energy storage unit supplies all of the energy to the critical load via the inverter. Therefore, the energy storage unit is required to supply rated power for at least 15 seconds, meaning that for a 1 MW rated UPS system at least 4.2 kWh of energy storage is required.
Consequently, a need exists for an innovation which will greatly reduce the duration of rated power required to be supplied by the energy storage unit and thus eliminate the need to use massive lead acid batteries in the energy storage unit of the UPS system.